1. Field of the Invention
The disclosures herein generally relate to a semiconductor stack and a vertical cavity surface emitting laser.
2. Description of the Related Art
A solid state laser such as Nd:GdVO4, Nd:YAG, etc., has a limited wavelength, whereas a semiconductor laser can emit various wavelengths of laser light because it is relatively easy to adjust the composition of active materials. Therefore, it is expected to be applied in fields requiring a high output laser. A vertical cavity surface emitting laser (VCSEL) has, in particular, a superior characteristic of wavelength controllability without suffering from mode hopping.
Such a semiconductor laser emits light of a predetermined wavelength at a specific band gap by current injection into an active layer or carrier injection with optical excitation. To implement carrier injection effectively, a quantum well active layer is widely used in an active layer structure. Moreover, to enable a higher output, a multiple quantum well (MQW) structure is generally used in which multiple quantum well layers are separated by barrier layers.
For example, in Non-patent document 1 discloses a structure in which three 8 nm-thick quantum well layers formed with GaInAsP are separated with 10 nm-thick barrier layers formed with GaInP.
Also, to achieve a higher output with a VCSEL, there are several requirements to be satisfied such as effective carrier confinement, improved gain, and a superior heat radiation characteristic for heat generated at active layers.
Patent document 1 discloses a structure which has improved carrier confinement effect by forming a layer with a broader band gap outside of barrier layers between which a quantum well layer is laminated, which is described as a carrier leak prevention layer.
Patent document 2 discloses an external reflecting mirror VCSEL using optical excitation to achieve a higher output. Specifically, in a structure disclosed in Patent document 2, GaInP layers with thickness of λ/2 (λ: an oscillating wavelength) are provided on both surfaces of a resonator to improve carrier confinement effect. Moreover, in the above structure, a GaInAs material having compressive strain is used for a quantum well layer, and a number of layers formed with a Ga(In)PAs material having tensile strain are used to compensate for the compressive strain.
However, the structure disclosed in Patent document 1 has a small band gap difference, with which it cannot be expected to achieve a higher output. Also, as the oscillation wavelength range of the semiconductor laser is 780 nm, Al0.2Ga0.8As is used for high refractive index layers at an upper semiconductor DBR (Distributed Bragg Reflector) and a lower DBR, and Al0.4Ga0.6As is used in upper cladding layers and lower cladding layers. However, it is not preferable to use these materials in a semiconductor laser if targeting a higher output, because these materials have low thermal conductivity.
Also, structures disclosed in Patent documents 2 and 3 use a GaInP layer or a layer formed with a Ga(In)PAs material. However, again, it is not preferable to use GaInP or a Ga(In)PAs material in a semiconductor laser if targeting a higher output, because these materials have low thermal conductivity. Moreover, when producing the structure disclosed in Patent document 2, if a Ga(In)PAs material is grown on GaAs, or GaAs is grown on a Ga(In)PAs material, it is necessary to change growth atmosphere from As atmosphere to P/As mixed atmosphere, or from P/As mixed atmosphere to As atmosphere. In such cases, As or P atoms tend to become detached, which generates lattice defects at the interface, increases absorption of laser light, and prevents a higher output.
Non-patent document 3 discloses VCSELs with various structures for optical excitation. As an example, a structure is disclosed in which five layers are laminated in which each of the five layers includes a pair of 2QW quantum well layers and a carrier block layer in-between the pair of 2QW quantum well layers. However, it cannot be expected to achieve effective carrier confinement effect because heights of barrier layers are all the same. Also, the 2QW quantum well layers are evenly disposed, in which an active layer structure for optical excitation cannot be recognized. Another feature is that the number of quantum well layers in an MQW active layer is increased when moving towards the surface to prevent carrier overflow at active layers close to the surface. However, it is not a suitable structure for a higher output because, again, heights of barrier layers in a carrier block layer between MQMs are all the same.
Also, Non-patent document 4 discloses a structure in which the number of quantum wells is increased when moving towards the surface. Again, heights of barrier layers in a carrier block layer are all the same.
Also, Non-patent document 5 discloses a structure of an optical-excitation VCSEL wafer using a red-light emitting material. In general, it is difficult to use a red-light emitting AlGaInP material to achieve a higher output because it cannot obtain a sufficient amount of band offset with a barrier material. Here, although a number of quantum wells are laminated to achieve a certain level of output, it is difficult to achieve a higher output because low carrier confinement effect inherent to the structure remains unchanged.
Also, Non-patent document 6 discloses a structure in which a reflection prevent layer, also used as a carrier block layer, is formed with Al0.3Ga0.7As at the outermost surface. However, there is a high likelihood that a number of carriers susceptible to non-light-emitting recombination are generated, because there is no carrier block layer between an active layer and a reflecting mirror, the active layer is attached to a low refractive index layer Al0.8Ga0.2As in the reflecting mirror, and layers with a high Al composition include much oxygen. Also, the structure described in Non-patent document 6 has low thermal conductivity because Al0.8Ga0.2As is used in the low refractive index layer in the reflecting mirror as above, with which a higher output cannot be expected due to low thermal conductivity.
Also, in Patent document 4, although there is a description on a carrier block layer, heights of barriers are all the same, and carrier density distribution relevant to optical excitation is not taken into account at all.
Patent Documents:
    1. Japanese Laid-open Patent Application No. 2000-174329    2. Japanese Laid-open Patent Application (Translation of PCT Application) No. 2002-523889    3. Japanese Patent No. 4837830    4. U.S. Pat. No. 5,461,637Non-Patent Documents:    1. IEEE JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL 22, No. 12, 2004, p 2828-2833    2. J. W. Matthews and A. E. Blakslee: J. Cryst. Growth 27 (1974) 118    3. Proc. of SPIE Vol. 7919 791903 p 1-11    4. Physics Report 429 (2006) p 67-120    5. Proc. of SPIE Vol. 7919 791908 p 1-10    6. IEEE. Photonic Technology Letters, Vol. 9, No. 8 (1997) p 1063-1065